1. Field of the Invention
The present invention relates to a non-contact type tonometer for obtaining the eye pressure of an eye to be examined.
2. Description of the Related Art
Conventionally, a non-contact type tonometer is known in which a stream of air is blown onto an eye to be examined to deform the cornea, and the degree of the deformation thereof is optically detected to measure the eye pressure of the examined eye without contact.
In a non-contact type tonometer of this kind, its optical system is readily stained with tears, dusts, and the like, because the stream of air is blown via a nozzle to the examined eye. The stained optical system sometimes precludes the alignment thereof, or produces errors in the measurements. In order to solve this problem, Japanese Patent Gazette No. Hei 2-12086 (Prior Art 1) and Japanese Patent Laid-Open Publication No. Hei 7-171110 (Prior Art 2) disclose a non-contact type tonometer comprising a mechanism for detecting stains in the optical system and alerting thereon an examiner.
In Prior Art 1, a lens cap is mounted in front of an objective lens when the detection of the stains is performed. The stain can be detected by measuring the quantity of the reflected light from the lens cap.
However, it is very bothersome for the operator to mount the lens cap in front of the objective lens every time he or she begins detecting the stain.
Meanwhile, in Prior Art 2, a stain-detecting system detects the stain based on the quantity of the alignment detection light reflected at the cornea. The operator does not have to do anything particular in order to detect the stain, such as mounting a lens cap in front of the objective lens.
However, in the apparatus disclosed in Prior Art 2, another problem occurs when the operation check of the instrument is carried out by using a model eye. If the reflectance of the model eye is much smaller than average ones of human eyes, the stain-detecting system may judge that the stain exists, even though the optical system is not stained.
The reason is explained below.
The apparatus disclosed in Prior Art 2 judges that the stain exists, when the quantity of the cornea-reflected light of the alignment detection light becomes equal to or smaller than the prescribed value. And this "prescribed value" is determined based on the average reflectance of human eyes.
On the other hand, the reflectance of a model eye depends on its material and structure. Some model eyes have reflectance closer to the average reflectance of human eyes while others have reflectance much smaller than the average reflectance of human eyes.
Thus, when a model eye having extremely small reflectance is used, the stain alert is issued even in unstained optical systems, because the "prescribed value" is determined based on the average reflectance of human eyes.